Temperature sensors are widely used in various fields, such as, for example, in automation, or for medical applications or also in devices installed on wearable supports for continuously monitoring patients.
These so-called smart temperature sensors are widely applied in PCs and laptops to monitor the temperature of the microprocessor, the case, and power-consuming peripheral ICs. This application requires low-cost temperature sensors with a relatively coarse temperature resolution, the analog output of which is converted into a digital signal with an on-chip analog-to-digital converter, that provides the temperature information in digital form to digital blocks of the chip for allowing thermal management of the device.
The article of A. Bakker “CMOS Smart Temperature Sensors—An Overview”, Proceedings of IEEE Sensors, Vol. 2, pages 1423-1427, 2002, provides a comprehensive overview of CMOS smart temperature sensors. They are substantially based on directly-biased semiconductor PN junctions having two transistors thermally in contact with the element whose temperature is to be sensed. These temperature sensors provide, in ideal conditions, that is when the reverse saturation currents of the two transistors are precisely the same, an analog sense voltage that is directly proportional to the absolute temperature of the junctions.
Unfortunately, process spread causes significant differences between the reverse saturation currents of the transistors, thus the generated analog sense voltage may depend from temperature with a law that greatly differs in an unknown fashion from the expected (proportionality) relation. This source of error can be reduced by using a single transistor twice: two constant currents with a known ratio are generated by an external circuit. However there is still the problem of weak sensitivity (<100 uV/C*log n(ratio of currents)) which creates the need for a low-noise amplification chain that may be expensive. Moreover, the sense voltage should be converted into digital form in order to be provided in input to a microcontroller, thus an analog-to-digital converter is needed, with consequent increase of costs, power consumption, and further worsening of the accuracy of the digital information received by the microcontroller.
For these reasons, temperature sensors based upon PN junctions may be considered intrinsically coarse (typical sensitivity 2-3 mV/° C. with a single transistor used once, <100 uV/C with a single transistor used twice, typical accuracy ±0.1C) and with considerable power consumption and complexity.
In an attempt to devise new temperature sensors with enhanced properties, the present trend of research is pointing towards temperature sensors that do not use PN junctions, such as thermistors or resistance temperature detectors (RTD), that are characterized by refined sensitivity and accuracy.
Unfortunately, accurate RTDs and thermistors are relatively expensive, thus they may not be adequate for providing disposable temperature sensors destined to be installed on wearable supports for monitoring patients. Moreover, they provide a temperature sense signal in analog form, thus an analog-to-digital converter is still required.